1. Field of the Invention
The invention relates to improved analog-to-digital conversion circuits, and especially to circuitry for improving the resolution or accuracy of analog-to-digital converter circuits.
2. Brief Description of the Prior Art
Frequently, prior art A/D converters require connection of numerous external capacitors, resistors, and diodes. Frequently, such external components must be carefully matched and of high precision, and are frequently highly temperature sensitive, and frequently have characteristics which change with age, thereby necessitating frequent manual recalibration of the A/D converter.
An improved prior art analog-to-digital converter which overcomes many of these difficulties is described in copending application Ser. No. 666,528, filed Mar. 12, 1976, by Robert C. Huntington and assigned to the present assignee and incorporated herein by reference. In the co-pending Huntington application, amplifier offset voltages are compensated for automatically and a zero reference is established automatically twice every conversion cycle to provide a corrected output. A compensation capacitor and an integrating capacitor are selectively charged. A reference signal is then integrated to measure the magnitude of the difference between an integrator offset voltage and a comparator threshold voltage. A counter counts during the integration to produce a digital representation of the difference between the integrator offset voltage and the comparator threshold, which digital representation is then stored. The compensation capacitor and the integrating capacitor are again selectively charged. The integrating capacitor is charged to a voltage representative of the analog input signal. During integration of the reference signal from the voltage representative of the analog input signal to the comparator threshold voltage, the counter continually counts, but is automatically reset to zero and continues counting when the count reaches the value of the stored number which represents the difference between the integrator offset voltage and the comparator threshold voltage. However, the accuracy or resolution of the least significant digit of the contents of the counter at the end of the second integration has an uncertainty associated therewith, which uncertainty is the sum of the uncertainty caused by the fact that the portions of the basic operating cycle of the analog-to-digital converter at which both the first and the second integrations occur are undetermined.